This invention relates to a method of pre-heating particles of a hydrocarbon-bearing substrate, for example an oil shale, tar sand or a bituminous coal.
It is well known that hydrocarbons can be extracted from such hydrocarbon-bearing substrates by heating particles of the substrate at a temperature of at least 400.degree. C. in the substantial absence of oxygen, and recovering the liberated hydrocarbons. In the case of oil shale this process is usually referred to as retorting and, in the case of bituminous coal, is called pyrolysis.
In a number of different known processes the heating of the substrate particles is carried out by heat exchange with a heat-bearing medium. Such a heat-bearing medium may, for example, be a solid medium consisting of inert particles which are heated in a separate vessel and then circulated through the extraction vessel.
Certain of the known retorting processes make use of the fact that the spent substrate, i.e. the substrate after extraction of the hydrocarbons, may contain appreciable amounts of coke. It has therefore been proposed to generate the heat required for the retorting process by complete or partial combustion of this coke to produce a hot spent substrate. This hot spent substrate may be employed as heat-bearing medium for the extraction process.
It is desirable that the substrate particles used in such an extraction process have been subjected to a separate pre-heating step. This pre-heating step essentially involves heating the substrate particles to a temperature below that at which the main extraction process takes place. Heat transfer to the substrate particles in the pre-heating step may be carried out by any suitable method, but it would be more advantageous if the heat required is taken from the hot spent substrate itself.